1. Field of the Invention
This invention is generally related to vertical axis windmills, or wind turbines, and is particularly related to a vertical axis wind turbine rotor structure.
2. Description of the Prior Art
A highly desirable feature for the rotor of a vertical-axis wind turbine is a sturdy resistance against centrifugal forces tending to bend the airfoil segments, and against gust moments tending to bend the vertical shaft. A major disadvantage of the conventional hoop-shaped Darrieus rotor is that it cannot be supported by a single main bearing at or near its center of mass; the otherwise severe shaft-bending moment is usually avoided by fastening the hoop segments to a hollow vertical tube which fits rotatably on a vertical pole having its upper end stabilized by guy wires slanting downward to ground anchors located far outside the rotor diameter.
One method of more efficiency utilizing the available ground space is to align coaxially in a suitable tower a vertical shaft extending upward through a tower-top thrust bearing to terminate in a balanced-Tee attachment to the underside of a long, horizontal beam having a vertical airfoil fitted at each end. However, this method has the disadvantage of generally severe centrifugal forces acting to bend the airfoils outward, unless the rotor speed and efficiency are reduced by cyclically weathervaning the airfoils. For example, in a 30 m.p.h. wind the centrifugal acceleration of non-weathervaned airfoils on a rotor of 50 ft. diameter is approximately 1936 ft. sec.sup.-2, or 60 times the acceleration of gravity. Sample calculations for this loading, using an 8.times.8 inch 6061-T6 aluminum I-beam as the airfoil spar, show that the cantilevered portion of the airfoil cannot be made longer than 7 feet without exceeding the 8000 psi unit-fiber yield strength.